This invention relates in general to photography equipment such as large, high precision cameras and deals more particularly with an improved track structure, usually overhead, for supporting large copying cameras and projectors.
In the graphic arts and reprographic fields, large cameras, projectors, and camera/projectors are used for negative making, plate making, print duplication, enlargements and other complicated functions such as lithographic press printing. The camera reproduces original material on a photosensitive medium, usually film. If the device functions as a projector, it projects the image (after the film has been processed) at its original size or at a different size if desired. Large cameras and projectors of this type are also used in reprographic work such as the preparation of engineering and architectural drawings and maps, and in other specialized areas such as seismic studies and printed circuit manufacturing. In these applications, the cameras and projectors are used to make intermediate photo tracings as well as end use (display) prints.
Modern cameras and projectors of this type are equipped with a subject holder which receives and holds the original drawings and other materials that are to be reproduced. In the projection mode of operation, the subject holder technically functions as an easel. The subject holder is located at one end of an overhead track having machined rails on which a wheeled carriage travels. The camera or projector is suspended from the carriage and can be moved toward and away from the subject holder by moving the carriage along the rails.
Any camera exposure requires a holder for sensitized material, an objective lens and an object (or subject holder). In focusing and sizing an image in the camera or projector relationship, it is always necessary to move at least two of the above three items with respect to the third item.
Traditionally, the film plane or lensboard is held stationary and the subject holder is nearly always one of the two components that is moved during the focusing and sizing of the image. Improved arrangements which involve fixedly locating the subject holder and moving the other optical components are shown in the patents to J. H. Wally, Jr. U.S. Pat. No. 3,639,054, issued Feb. 1, 1972; U.S. Pat. No. 3,645,621, issued Feb. 29, 1972; U.S. Pat. No. 3,724,948, issued Apr. 3, 1973; and U.S. Pat. No. 3,762,816, issued Oct. 2, 1973. Among the advantages of this type of arrangement is the ability to position the front and back lights in the most effective position and maintain the lights in such position without the need to move them or change their attitude.
Due to the size of many original drawings, the subject holder is often 12 feet long or longer. The need for such a large subject holder, coupled with the need to make size reductions with appropriate objective lenses, results in a track length of 30 feet or more in some cases.
In order to keep the floor as uncluttered as possible and to enhance the speed and safety of the operation, the support tracks are normally overhead structures which are supported only at the ends. A track supported only at the ends is highly susceptible to bending or sagging, particularly if it is a long track and is loaded with an optical compartment containing heavy components such as metal film holders, massive glass condensers, or in some cases, a moving subject holder and the lighting means associated therewith. The equipment which is borne by the track can weigh hundreds or even thousands of pounds, and this weight can deflect the track considerably, especially when the load is applied near the center of the track.
Bending of the overhead track under load is highly undesirable because it deflects the optical axis which extends through the center of the lens, thereby introducing distortion into the film image and detracting from the geometric accuracy of the reproduction. In this type of camera, accurate reproduction is of paramount importance because it is necessary to reproduce the original material with opposite borders parallel, corners square and free of unnecessary distortion in the field of the image.
In the past, various types of specially and massively constructed tracks have been proposed in attempts to counteract the heavy weights that are applied to the track and to negate the deflection of the optical components with respect to the optical axis. The long prevailing arrangement has been a welded track structure having rails which are machined to a straight and flat condition after construction of the track. The machine work is carried out in a plant on a large machine tool or mill. Although the rails can be machined to a precisely straight and flat condition in the mill, this construction does not compensate for the heavy loads that the rails must carry in actual use, and there is no way to simulate those loads during the milling procedure.
The track deflection varies both with the load and with the position of the load along the track. Near the center of the track span, the deflection is greatest. Near the ends of the track, the support posts or frames bear more of the load and the track deflection diminishes. None of the tracks which have been proposed in the past, once erect and in use, have been able to compensate for these complex effects, and the precision of the photographic or projection process has suffered accordingly. The overall stiffness of the overhead beams cannot, in and of itself, prevent deflection.